Acid solution fingering quantitative evaluation method for acid fracturing transformation of prepad fluid of geothermal reservoir of carbonate rock

Abstract

The invention relates to an acid solution fingering quantitative evaluation method for carbonate rock geothermal reservoir prepad fluid acid fracturing transformation, and belongs to the technical field of enhanced geothermal system reservoir excitation. A three-dimensional model and a multi-field coupling numerical model considering the fingering effect are established, the three-dimensional shape change of a fracture wall surface can be obtained by applying a numerical simulation method, and the acid liquid fingering effect in the carbonate rock thermal storage prepad fluid acid fracturing process is quantitatively evaluated by calculating an etching front edge fingering coefficient. The sensitivity of parameters such as the rock plate temperature, the prepad fluid viscosity and the acid injection displacement to the fingering effect is further analyzed, and a theoretical basis and technical support are provided for the geothermal reservoir deep acid fracturing transformation technology.

Description

Technical Field

[0001] The invention relates to an acid liquid fingering quantitative evaluation method for pre-fluid acid fracturing transformation of carbonate geothermal reservoirs, belonging to the technical field of enhanced geothermal system reservoir stimulation. Background Art

[0002] Enhanced geothermal systems can greatly improve geothermal extraction capacity and are an important means of large-scale utilization of geothermal resources. Reservoir stimulation is a key step. Conventional acid fracturing technology has a rapid acid consumption and limited range of action due to the excessively fast acid-rock reaction rate under high temperature environment. Deep acid fracturing technology is needed to achieve more profound transformation. Pre-fluid acid fracturing is one of the deep acid fracturing technologies and has been widely used in carbonate reservoirs. Its main principle is to first use a high-viscosity liquid as a pre-fluid to press the formation into cracks, and then inject acid to etch the crack wall. In this process, the low-temperature and low-viscosity acid will produce fingering when flowing in the high-viscosity pre-fluid. The more obvious the fingering effect is, the smaller the acid-rock contact area is, which slows down the acid consumption rate and is conducive to the migration of acid to deeper cracks. Therefore, it is of great significance to accurately and quantitatively evaluate the fingering effect of acid during the pre-fluid acid fracturing transformation of carbonate geothermal reservoirs.

[0003] At present, there are relatively few related studies. Chinese patent document CN118774768A discloses a method for evaluating the viscous fingering effect in acid fracturing fractures. This method uses a visualized flat-plate fracture device to quantitatively evaluate the viscous fingering flow effect. Chinese patent document CN116378631A discloses an acid fracturing parameter design method based on the evaluation of alternating injection fingering effect, providing the optimal design of acid fracturing fingering parameters. However, the existing technical achievements are mainly focused on the acid fingering evaluation of a single flow field, and ignore the influence of the high-temperature environment of the geothermal reservoir. When the acid contacts the high-temperature rock wall, a chemical reaction will occur, which will cause a series of complex multi-field coupling processes such as deformation of the wall morphology and evolution of the rock porosity and permeability relationship. Therefore, it is necessary to fully consider the coupling and linkage effects between acid fingering and other physical fields such as chemical reactions, heat transfer processes, and porosity and permeability changes. Summary of the invention

[0004] The purpose of the present invention is to solve the above-mentioned problems, provide a quantitative evaluation method for acid fingering in pre-fluid acid fracturing of carbonate geothermal reservoirs, establish a three-dimensional model and a multi-field coupling numerical model considering the fingering effect, and use a numerical simulation method to obtain the three-dimensional morphological changes of the fracture wall. By calculating the etching front fingering coefficient, the acid fingering effect in the process of pre-fluid acid fracturing of carbonate geothermal reservoirs is quantitatively evaluated. The sensitivity of parameters such as rock plate temperature, pre-fluid viscosity, and acid injection displacement to the fingering effect is also analyzed, providing a theoretical basis and technical support for deep acid fracturing technology of geothermal reservoirs.

[0005] The technical solution of the present invention is as follows: An acid fingering quantitative evaluation method for preflush acid fracturing of carbonate geothermal reservoirs, comprising the following steps: (1) Establish a three-dimensional model considering the multiphase flow fingering effect and a multi-field coupling numerical model. The multi-field coupling numerical model includes a porous medium seepage module, a chemical reaction module, a porous medium heat transfer module, and a mathematical calculation module; (2) Conduct numerical simulation experiments; obtain the etching results of high-temperature rock slabs under different conditions according to the experimental requirements for subsequent analysis; (3) Determine the fracture etching boundary of the rock slab after acid etching, and extract data from the points on the boundary; import the collected data into data processing software for subsequent calculations; (4) Data sorting and calculation; use MATLAB to calculate the etching front length L1 of the upper rock slab, the etching front length L2 of the lower rock slab, the total etching front length L0, and the fracture width D; (5) Calculate the fingering coefficient F of the etching front; this coefficient evaluates the strength of the fingering effect and the randomness of the distribution of the etching area on the fracture wall surface, and indirectly evaluates the degree of non-uniform etching of the fracture. The larger the coefficient, the longer the etching front length and the stronger the fingering effect. The smaller the coefficient, the closer the etching front is to the fracture width and the closer it is to the piston displacement process. At the same time, the fingering coefficient is also used to represent the degree of non-uniform etching of the fracture wall surface. The larger the coefficient, the more complex the etching front, the stronger the randomness of the distribution of the etching area, and the stronger the degree of non-uniform etching of the wall surface; (6) Conduct sensitivity factor analysis; based on the experimental results, analyze the influence of parameters such as rock slab temperature, preflush fluid viscosity, and acid injection displacement on the fingering coefficient of the etching front.

[0006] Preferably, in step (1), the three-dimensional model consists of parallel rock slabs of the same size distributed up and down, with a fracture area in the middle. The free tetrahedral network is used, and the subdivision method is the Delaunay method. The fracture network system is encrypted and constructed using a smaller grid size than the rock slab matrix.

[0007] Preferably, in step (1), the porous medium seepage module is used to describe the fluid flow process of the entire model. Among them, in the fracture area, the porous medium multiphase flow is followed, and the flow equation and the continuity equation are:

[0008]

[0009]

[0010]

[0011]

[0012] wherein, u p , u a are the fluid velocity vectors of the preflush fluid and the acid fluid respectively; , are the two-phase fluid pressures in the fracture; ▽ represents the gradient operator, i.e., grad, which is a mathematical operation symbol, k is the absolute permeability of the reservoir; k p (S), k a (S) are the permeabilities of the preflush fluid and the acid fluid respectively; k rp (S), k ra (S) are the relative permeabilities of the two-phase fluid respectively, and their values are affected by the acid fluid saturation S; , are the net exchange terms of the preflush fluid and the acid fluid between the media respectively; is the equivalent porosity of the fracture area; S p , S a are the saturations of the two-phase fluid respectively; In the slab matrix, the Darcy's law for porous media is followed, and the flow equation and the continuity equation are respectively:

[0013]

[0014] wherein, u m is the fluid velocity vector determined by Darcy's law in the porous media, , k are the matrix porosity and permeability respectively, is the net exchange term of the acid fluid between the fracture and the matrix, and the calculation method is as follows:

[0015] wherein, , are the fluid pressures in the fracture and the matrix respectively when approaching the fracture wall surface infinitely.

[0016] Preferably, in step (1), the chemical reaction module is used to describe the chemical reaction process of the whole model. Among them, the solute transport mass transfer and chemical reaction consumption equations in the fracture area are as follows:

[0017] wherein, d f is the fracture width, c f is the solute concentration in the fracture, R f is the solute reaction rate in the fracture, is the solute diffusion coefficient in the fracture, is the convection and diffusion exchange term of the solute between the fracture and the porous media, and the calculation method is:

[0018] In the formula: Indicates at the fracture wall surface; The mass balance equation for the solute transfer in the matrix in the liquid phase is:

[0019] In the formula, c a Is the concentration of acid in the liquid phase, c s Is the concentration of acid at the acid-rock interface, Is the diffusion coefficient of the solute in the matrix, Is the mass transfer coefficient, Is the specific surface area; Is the matrix porosity; Chemical reactions will dissolve the mineral components of carbonate rocks, resulting in an increase in the porosity of the rocks. The relationship between the porosity and chemical reactions is described by the continuity equation of the rock skeleton:

[0020] In the formula, ρ s Is the rock density, α represents the dissolution ability of acid, a v Is defined as the mass of solid that can be dissolved per mole of acid reaction, R(c s ) is the reaction kinetics, Is the matrix porosity.

[0021] Preferably, in step (1), the porous medium heat transfer module is used to describe the heat transfer process of the entire model, including heat convection and heat conduction; The heat transfer equation considering heat convection and heat conduction in the fracture is:

[0022]

[0023]

[0024] In the formula: T is the temperature, Is the equivalent density of the fracture, , Are the acid solution density and the preflush fluid density respectively, Is the equivalent heat volume of the fracture, Is the specific heat capacity of the rock, Is the specific heat capacity of the acid solution, Is the specific heat capacity of the preflush fluid, Is the equivalent heat transfer coefficient of the fracture, Is the heat transfer coefficient of the rock, Is the heat transfer coefficient of the acid solution, is the heat transfer coefficient of the preflush fluid, is the heat exchange between two media.

[0025] Considering heat convection, heat conduction, and the heat effect of chemical reactions in the matrix, its energy conservation equation is:

[0026]

[0027]

[0028]

[0029] In the formula: is the equivalent density of the matrix, is the equivalent heat capacity of the matrix, is the equivalent heat transfer coefficient of the matrix, is the heat release of the chemical reaction, H is the enthalpy change, is the heat exchange between two media, and the calculation method is:

[0030] In the formula: represents at the fracture wall surface.

[0031] Preferably, in step (1), the mathematical calculation module is used to describe the evolution of the pore - permeability relationship of the rock matrix part. The pore - permeability change will further affect other physical processes. The calculation formula for the relationship between permeability, pore radius, specific surface area, and porosity is:

[0032]

[0033]

[0034] In the formula, and are the porosity and initial porosity of the rock respectively; and are the permeability and initial permeability of the rock respectively; and are the pore radius and initial pore radius of the rock respectively; and are the specific surface area and initial specific surface area of the rock respectively; is a constant related to the pore structure; By coupling and linking the variables between multiple physical - field control equations, a numerical model of multi - field coupling of heat - water - chemistry at the core scale is finally constructed.

[0035] Preferably, in step (2), different experimental conditions are set to carry out parameter sensitivity analysis. To explore the influence law of slab temperature on the etching front fingering coefficient, five different initial temperature values are set between 353K / 80°C and 453K / 180°C. Other factors are fixed, and the acid etching process at different temperatures is simulated within the same time; To explore the influence law of the preflush fluid viscosity on the etching front fingering coefficient, five graduation values are selected between 20 mPa·s and 60 mPa·s. Other factors are fixed, and the acid etching process under different viscosity differences is simulated within the same time; To explore the influence law of the acid injection displacement on the etching front fingering coefficient, from 2 mL·min -1 to 6 mL·min -1 five graduation values are selected between them. Other factors are fixed, and the acid etching process at different displacements is simulated within the same time.

[0036] Preferably, in step (3), the method for determining the etching boundary is to indirectly calculate the fracture width by using the area with relatively high porosity in the porosity distribution, rather than directly calculating the fracture width. The calculation method is: , where is the porosity of the etching boundary, is the coefficient determined according to the experiment, is the maximum value of the porosity set in the numerical model, taking 0.99, and taking the isosurface with porosity of can determine the etching boundary.

[0037] Preferably, in step (3), data extraction is to extract the three-dimensional coordinates of all discrete points on the fracture boundary with the help of the post-processing module of the numerical simulation software. The data processing software is software that can implement data screening and sorting functions, such as Excel, Origin, etc.

[0038] Preferably, in step (4), the etching front is defined as the part of the etching boundary curve at the position equal to the height of the original fracture wall on the etching boundary; The calculation methods for the upper slab etching front length L1 and the lower slab etching front length L2 are to screen the points that meet the definition of the fingering front among all the point coordinates on the fracture boundary, draw the etching front curve, export the etching front curve, and use MATLAB to calculate the curve length; The total etching front length L0 is: L0 = L1 + L2.

[0039] Preferably, in step (5), the etching front fingering coefficient is defined as the ratio of the length of the etching front to the fracture width; The calculation formula for the etching front fingering coefficient F is: .

[0040] The beneficial effects of the present invention are as follows: 1. Numerical simulation is scientific and accurate, and more in line with the actual situation: The thermo-hydro-chemical (THC) multi-field coupling numerical model used in the present invention introduces the multiphase flow theory, considers the fingering flow effect of low-temperature and low-viscosity acid solution in fractures in the presence of a high-viscosity preflush fluid, and simultaneously simulates a series of physical processes such as the chemical etching reaction after acid-rock contact, heat transfer process, and evolution of pore permeability relationship. The simulation results are more scientific and accurate.

[0041] 2. Digital processing of the surface morphology before and after acid etching of fractures: With the help of the post-processing module of the numerical simulation software in the present invention, the three-dimensional coordinates of all discrete points on the fracture boundary are extracted, and data screening is realized using data processing software to accurately obtain the data of the etching front boundary.

[0042] 3. Quantitative evaluation of the acid fingering effect in high-temperature carbonate reservoirs: The present invention defines the fingering coefficient of the etching front, and reflects the fingering effect through the quantitative evaluation of the etching morphology on the fracture wall surface.

[0043] 4. Multi-factor sensitivity analysis: Based on the simulation and data processing results, the present invention clarifies the influence of parameters such as slab temperature, preflush fluid viscosity, and acid injection rate on the fingering coefficient of the etching front, providing a theoretical basis and technical support for the optimization of process parameters for preflush acid fracturing in carbonate reservoirs. Description of the Drawings

[0044] Figure 1 is the process flow chart of the present invention; Figure 2 is the three-dimensional model diagram of the present invention, where A is the injection end and B is the outlet end; Figure 3 is the diagram for determining the fracture etching boundary from the porosity field of the present invention, where Figure 3 in (a) is the global porosity distribution diagram; Figure 3 in (b) is the high-porosity area distribution diagram; Figure 3 in (c) is the schematic diagram of the etching boundary; Figure 4 is the curve diagram for extracting the etching front from the fracture etching boundary of the present invention; Figure 5 is the variation diagram of the fingering coefficient of the etching front of the present invention with the influence of different factors, where Figure 5 in (a) is the variation diagram of the influence of slab temperature on the fingering coefficient of the etching front; Figure 5 in (b) is the variation diagram of the influence of preflush fluid viscosity on the fingering coefficient of the etching front; Figure 5 in (c) is the variation diagram of the influence of acid injection rate on the fingering coefficient of the etching front. Detailed Embodiments

[0045] The present invention will be further described below by way of examples in conjunction with the accompanying drawings, but is not limited thereto.

[0046] In an embodiment of the present invention: Numerical simulation uses COMSOL Multiphysics multi-physics simulation software; Data processing and analysis are carried out using MATLAB and Origin software.

[0047] Example 1: As Figure 1 shown, this embodiment provides a quantitative evaluation method for acid fingering in the preflush acid fracturing of carbonate geothermal reservoirs, including the following steps: (1) Establish a three-dimensional model considering the multiphase flow fingering effect and a multi-field coupling numerical model. The multi-field coupling numerical model includes a porous medium seepage module, a chemical reaction module, a porous medium heat transfer module, and a mathematical calculation module; Use COMSOL Multiphysics multi-physics simulation software to establish a three-dimensional model. The three-dimensional model is composed of parallel rock slabs of the same size distributed up and down to hold the middle fracture area. The computational domain size of a single rock slab is a three-dimensional grid of 10 cm × 2 cm × 1 cm, and the fracture domain size is 10 cm × 2 cm × 0.1 cm; Use a free tetrahedral network, the subdivision method is the Delaunay method, and the fracture network system is encrypted using a smaller grid size than the rock slab matrix, as Figure 2 shown.

[0048] The porous medium seepage module is used to describe the fluid flow process of the entire model. Among them, in the fracture area, porous medium multiphase flow is followed, and the flow equation and continuity equation are:

[0049]

[0050]

[0051]

[0052]

[0053] Among them, u p , u a are the fluid velocity vectors of the preflush fluid and the acid fluid, respectively; , are the two-phase fluid pressures in the fracture; ▽ represents the gradient operator, that is, grad, which is a mathematical operation symbol, k is the absolute permeability of the reservoir; k p (S), k a(S) is the permeability of the preflush fluid and the acid fluid; k rp (S), k ra (S) are the relative permeabilities of the two-phase fluid, and their values are affected by the acid fluid saturation S; , are the net exchange terms between the preflush fluid and the acid fluid in the medium; is the equivalent porosity of the fracture area; S p , S a are the saturations of the two-phase fluid; In the slab matrix, the Darcy's law for porous media is followed, and the flow equation and the continuity equation are respectively:

[0054]

[0055] where, u m is the fluid velocity vector determined by Darcy's law in the porous medium, , k are the matrix porosity and permeability respectively, is the net exchange term between the acid fluid in the fracture and the matrix, and the calculation method is as follows:

[0056] where, , are the fluid pressures in the fracture and the matrix respectively when approaching the fracture wall infinitely.

[0057] The chemical reaction module is used to describe the chemical reaction process of the whole model. Among them, the transport and mass transfer and chemical reaction consumption equations of the solute in the fracture area are as follows:

[0058] where, d f is the fracture width, c f is the concentration of the solute in the fracture, R f is the reaction rate of the solute in the fracture, is the diffusion coefficient of the solute in the fracture, is the convection and diffusion exchange term between the solute in the fracture and the porous medium, and the calculation method is:

[0059] In the formula: represents at the fracture wall surface; The material balance equation for the transport of the solute in the matrix in the liquid phase is:

[0060] In the formula, c ac is the concentration of acid in the liquid phase s is the concentration of acid at the acid-rock interface is the diffusion coefficient of the solute in the matrix is the mass transfer coefficient is the specific surface area is the matrix porosity Chemical reactions will dissolve the carbonate rock mineral components, resulting in an increase in the porosity of the rock. The relationship between porosity and chemical reactions is described by the continuity equation of the rock skeleton:

[0061] In the formula, ρ s is the rock density, α represents the dissolution ability of acid, and a v is defined as the mass of solid that can be dissolved per mole of acid reaction. R(c s ) is the reaction kinetics is the matrix porosity

[0062] The heat transfer module of the porous medium is used to describe the heat transfer process of the entire model, including heat convection and heat conduction The heat transfer equation considering heat convection and heat conduction in the fracture is:

[0063]

[0064]

[0065] In the formula: T is the temperature is the equivalent density of the fracture , are the acid solution density and the preflush fluid density respectively is the equivalent heat capacity of the fracture is the specific heat capacity of the rock is the specific heat capacity of the acid solution is the specific heat capacity of the preflush fluid is the equivalent heat transfer coefficient of the fracture is the heat transfer coefficient of the rock is the heat transfer coefficient of the acid solution is the heat transfer coefficient of the preflush fluid is the heat exchange between the two media

[0066] Considering heat convection, heat conduction, and chemical reaction heat effects in the matrix, its energy conservation equation is:

[0067]

[0068]

[0069]

[0070] In the formula: is the equivalent density of the matrix, is the equivalent heat volume of the matrix, is the equivalent heat transfer coefficient of the matrix, is the heat release of the chemical reaction, and H is the enthalpy change, is the heat exchange between two media, and the calculation method is:

[0071] In the formula: represents at the fracture wall surface.

[0072] In step (1), the mathematical calculation module is used to describe the evolution of the pore permeability relationship of the rock matrix part. The pore permeability change will further affect other physical processes. The relationship calculation formula between permeability, pore radius, specific surface area and porosity is:

[0073]

[0074]

[0075] In the formula, and are the porosity and initial porosity of the rock respectively; and are the permeability and initial permeability of the rock respectively; and are the pore radius and initial pore radius of the rock respectively; and are the specific surface area and initial specific surface area of the rock respectively; is a constant related to the pore structure; By coupling and linking the variables between multiple physical field control equations, a numerical model of multi-field coupling of heat-water-chemistry at the core scale is finally constructed.

[0076] (2) Conduct numerical simulation experiments; obtain the etching results of high-temperature rock slabs under different conditions according to the experimental requirements for subsequent analysis; Setting different experimental conditions is for carrying out parameter sensitivity analysis. To explore the influence law of the rock slab temperature on the etching front fingering coefficient, 5 different initial temperature values (80°C, 105°C, 130°C, 155°C, 180°C) are set between 353K / 80°C and 453K / 180°C. Fixing other factors, simulate the acid etching process at different temperatures within the same time; To explore the influence law of the viscosity of the preflush fluid on the fingering coefficient of the etching front, five graduation values (20 mPa·s, 30 mPa·s, 40 mPa·s, 50 mPa·s, 60 mPa·s) were selected between 20 mPa·s and 60 mPa·s. Other factors were fixed, and the acid etching process under different viscosity differences within the same time was simulated; To explore the influence law of the injection displacement of the acid fluid on the fingering coefficient of the etching front, from 2 mL·min -1 to 6 mL·min -1 five graduation values (2 mL·min -1 , 3 mL·min -1 , 4 mL·min -1 , 5 mL·min -1 , 6 mL·min -1 ) were selected. Other factors were fixed, and the acid etching process under different displacements within the same time was simulated. The results are shown in Table 1.

[0077] Table 1: Reaction conditions corresponding to the rock slab numbers

[0078] (3) Determine the fracture etching boundary of the rock slab after acid etching, and extract data for the points on the boundary; import the collected data into data processing software for subsequent calculations; The method for determining the etching boundary is to indirectly calculate the fracture width using the region with relatively high porosity in the porosity distribution instead of directly calculating the fracture width. The calculation method is: , where is the porosity of the etching boundary, is the coefficient determined according to the experiment, is the maximum value of the porosity set in the numerical model, taking 0.99, and the isosurface with a porosity of can be used to determine the etching boundary.

[0079] Data extraction means that with the help of the post-processing module of the numerical simulation software, the three-dimensional coordinates of all discrete points on the fracture boundary are extracted. The data processing software is Origin, which can implement data screening and sorting functions, as Figure 3 shown.

[0080] (4) Data sorting and calculation; use MATLAB to calculate the etching front length L1 of the upper rock slab, the etching front length L2 of the lower rock slab, the total etching front length L0, and the fracture width D; The etching front is defined as the part of the etching boundary curve at the position equal to the height of the original fracture wall on the etching boundary; The calculation methods for the etching front length L1 of the upper rock slab and the etching front length L2 of the lower rock slab are as follows: Screen all the point coordinates on the fracture boundary, draw the etching front curve for the points that meet the definition of the fingering front, export the etching front curve, and use MATLAB to calculate the curve length. As Figure 4 shown; The total length L0 of the etching front is: L0 = L1 + L2.

[0081] (5) Calculate the fingering coefficient F of the etching front; this coefficient evaluates the strength of the fingering effect and the randomness of the distribution of the etching area on the fracture wall surface, and indirectly evaluates the degree of non-uniform etching of the fracture. The larger the coefficient, the longer the etching front length and the stronger the fingering effect. The smaller the coefficient, the closer the etching front is to the fracture width and the closer it is to the piston displacement process. At the same time, the fingering coefficient is also used to represent the degree of non-uniform etching of the fracture wall surface. The larger the coefficient, the more complex the etching front, the stronger the randomness of the distribution of the etching area, and the stronger the degree of non-uniform etching of the wall surface; The fingering coefficient of the etching front is defined as the ratio of the length of the etching front to the fracture width; The calculation formula for the fingering coefficient F of the etching front is: .

[0082] (6) Conduct a sensitivity factor analysis; based on the experimental results, analyze the influence of parameters such as the rock slab temperature, the viscosity of the preflush fluid, and the acid injection displacement on the fingering coefficient of the etching front. The results are as Figure 5 shown. It can be seen that the non-uniform etching coefficient has a certain linear correlation with the rock slab temperature, the viscosity of the preflush fluid, and the acid injection displacement.

[0083] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above-described exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary rather than restrictive. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be embraced within the present invention. Any reference signs in the claims should not be regarded as limiting the claimed rights.

[0084] In addition, it should be understood that although this specification is described in terms of embodiments, not every embodiment only contains an independent technical solution. This narrative manner of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in the embodiments can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A quantitative evaluation method for acid fingering of pre-fluid acid fracturing of carbonate geothermal reservoirs, characterized in that: The steps include: (1) Establish a three-dimensional model and a multi-field coupling numerical model that considers the multiphase flow fingering effect. The multi-field coupling numerical model includes a porous media seepage module, a chemical reaction module, a porous media heat transfer module, and a mathematical calculation module; (2) Conduct numerical simulation experiments to obtain high-temperature rock plate etching results under different conditions according to experimental requirements; (3) Determine the crack etching boundary of the rock plate after acid etching, extract data from the points on the boundary; import the collected data into the data processing software; (4) Data collation and calculation: Use MATLAB to calculate the upper slab etching front length L1, the lower slab etching front length L2, the total etching front length L0, and the crack width D; (5) Calculate the etching front edge index F; The larger the coefficient, the longer the etching front is and the stronger the fingering effect is. The smaller the coefficient, the closer the etching front is to the crack width and the closer it is to the piston displacement process. At the same time, the fingering coefficient is also used to indicate the degree of non-uniform etching on the crack wall. The larger the coefficient, the more complex the etching front is, the stronger the randomness of the etching area distribution is, and the stronger the degree of non-uniform etching on the wall is. (6) Conduct sensitive factor analysis; based on the experimental results, analyze the effects of rock plate temperature, pre-fluid viscosity, and acid injection rate on the etching front fingering coefficient.

2. The method for quantitative evaluation of acid fingering for acid fracturing of carbonate geothermal reservoir pre-fluid according to claim 1, characterized in that: In step (1), the three-dimensional model is composed of parallel rock plates of the same size distributed up and down, with a fracture area in the middle.

3. The method for quantitative evaluation of acid fingering for acid fracturing of carbonate geothermal reservoir pre-fluid according to claim 2, characterized in that: In step (1), the porous media seepage module is used to describe the fluid flow process of the entire model, where the porous media multiphase flow is followed in the fracture area, and the flow equation and continuity equation are: Among them, u p ,u a are the fluid velocity vectors of the pre-pad fluid and the acid fluid, respectively; , are the two-phase fluid pressures in the fracture respectively; ▽ represents the gradient operator, k is the absolute permeability of the reservoir; k p (S), k a (S) are the permeabilities of the pre-fluid and the acid solution, respectively; k rp (S), k ra (S) are the relative permeabilities of the two-phase fluids; , are the net exchange terms of the pre-fluid and the acid solution between the media, respectively; is the equivalent porosity of the fracture area; S p , S a are the saturations of the two-phase fluids, respectively; In the rock matrix, the porous medium Darcy's law is followed, and the flow equation and continuity equation are: Among them, u m is the fluid velocity vector determined by Darcy’s law in porous media, , k are matrix porosity and permeability, is the net exchange term of acid between fracture and matrix, and is calculated as follows: in, , They are the fluid pressures in the crack and matrix when they are infinitely close to the crack wall.

4. The method for quantitative evaluation of acid fingering for acid fracturing of carbonate geothermal reservoir pre-fluid according to claim 3, characterized in that: In step (1), the chemical reaction module is used to describe the chemical reaction process of the entire model, where the solute migration, mass transfer and chemical reaction consumption equations in the fracture area are as follows: Among them, d f is the crack width, c f is the concentration of solute in the crack, R f is the reaction rate of the solute in the crack, is the diffusion coefficient of the solute in the crack, is the convection and diffusion exchange term of solute between fracture and porous medium, and the calculation method is: Where: Represents the crack wall; The material balance equation for the transport of solutes in the matrix in the liquid phase is: In the formula, c a is the concentration of acid in the liquid phase, c s is the concentration of acid at the acid-rock interface, is the diffusion coefficient of the solute in the matrix, is the mass transfer coefficient, is the specific surface area; is the matrix porosity; Chemical reactions will dissolve carbonate mineral components, causing the porosity of the rock to increase. The relationship between porosity and chemical reactions is described by the continuity equation of the rock skeleton: In the formula, ρ s is the rock density, α represents the solubility of the acid, a v Defined as the mass of solid that can be dissolved per mole of acid reaction, R (c s ) is the reaction power, is the matrix porosity.

5. The method for quantitative evaluation of acid fingering for acid fracturing of carbonate geothermal reservoir pre-fluid according to claim 4, characterized in that: In step (1), the porous media heat transfer module is used to describe the heat transfer process of the entire model, including heat convection and heat conduction; The heat transfer equation considering heat convection and heat conduction in the crack is: Where: T is temperature, is the equivalent density of the crack, , are the density of acid solution and the density of pre-fluid, is the equivalent heat capacity of the crack, is the specific heat capacity of rock, is the specific heat of the acid solution, is the specific heat capacity of the pre-liquid, is the equivalent heat transfer coefficient of the crack, is the heat transfer coefficient of rock, is the heat transfer coefficient of the acid solution, is the pre-fluid heat transfer coefficient, For heat exchange between two media; Considering the thermal convection, thermal conduction and chemical reaction thermal effects in the matrix, the energy conservation equation is: Where: is the equivalent density of the matrix, is the matrix equivalent heat capacity, is the matrix equivalent heat transfer coefficient, is the heat released by the chemical reaction, H is the enthalpy change, is the heat exchange between the two media, and the calculation method is: Where: Represents the crack wall.

6. The method for quantitative evaluation of acid fingering for acid fracturing of carbonate geothermal reservoir pre-fluid according to claim 5, characterized in that: In step (1), the mathematical calculation module is used to describe the evolution of the porosity-permeability relationship of the rock matrix. The porosity-permeability change will further affect other physical processes. The relationship between permeability, pore radius, specific surface area and porosity is calculated as follows: In the formula, and are the porosity and initial porosity of the rock, respectively; and are the permeability and initial permeability of the rock, respectively; and are the pore radius and initial pore radius of the rock respectively; and are the specific surface and initial specific surface of the rock, respectively; is a constant related to the pore structure; By coupling and linking the variables among multiple physical field control equations, we can eventually build a core-scale thermal-hydraulic-chemical multi-field coupled numerical model.

7. The method for quantitative evaluation of acid fingering for acid fracturing of carbonate geothermal reservoir pre-fluid according to claim 6, characterized in that: In step (2), different experimental conditions are set to carry out parameter sensitivity analysis. In order to explore the influence of the rock plate temperature on the etching front fingering coefficient, five different initial temperature values ​​are set from 353K / 80℃ to 453K / 180℃, and other factors are fixed to simulate the acid etching process at different temperatures in the same time. In order to explore the influence of the viscosity of the pre-fluid on the fingering coefficient of the etching front, 5 scale values ​​were selected from 20 mPa·s to 60 mPa·s, and other factors were fixed to simulate the acid etching process under different viscosity differences in the same time. In order to explore the influence of acid injection rate on the etching front fingering coefficient, the injection rate was changed from 2 mL·min -1 to 6 mL·min -1 Five graduation values ​​were selected, and other factors were fixed to simulate the acid etching process under different displacements within the same time.

8. The method for quantitative evaluation of acid fingering for acid fracturing of carbonate geothermal reservoir pre-fluid according to claim 7, characterized in that: In step (3), the etching boundary is determined by indirectly calculating the crack width using the relatively high porosity region in the porosity distribution. The calculation method is: ,in, is the porosity of the etched boundary, is the coefficient determined by experiment, The maximum porosity value set in the numerical model is 0.99, and the porosity is The isosurface of can determine the etching boundary; The data extraction is to extract the three-dimensional coordinates of all discrete points on the fracture boundary, and the data processing software is software for realizing data screening and sorting functions.

9. The method for quantitative evaluation of acid fingering for acid fracturing of carbonate geothermal reservoir pre-fluid according to claim 8, characterized in that: In step (4), the etching front is defined as the portion of the etching boundary curve at the same height as the original crack wall surface on the etching boundary; The calculation method of the etching front length L1 of the upper rock plate and the etching front length L2 of the lower rock plate is to select all the points on the fracture boundary that meet the definition of the finger front to draw the etching front curve, derive the etching front curve, and use MATLAB to calculate the curve length; The total length L0 of the etching front edge is: L0= L1+ L2.

10. The method for quantitative evaluation of acid fingering for acid fracturing of carbonate geothermal reservoir pre-fluid according to claim 9, characterized in that: In step (5), the etching front fingering coefficient is defined as the ratio of the length of the etching front to the crack width; The calculation formula of etching front edge index F is: .

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